Toxicologists monitoring the American food supply for traces of melamine after it was found in imported ingredients in the contaminated pet food that has killed at least 16 dogs and cats and sickened thousands of others said yesterday that even if there were small amounts of it in the American food supply, it would be unlikely to pose much of a threat to humans.

Sampling thus far by the Food and Drug Administration for melamine, which has also been detected in chicken feed on some farms in Indiana, has not turned up the chemical in food meant for humans, and the trace amounts found in some poultry feed — and hog feed — would presumably be excreted or broken down by the animals before they were slaughtered, scientists and federal officials said.

Also, a survey of poison control centers, veterans’ hospitals and a sample of private hospitals across the country by the federal Centers for Disease Control and Prevention has found no increase in reports of kidney diseases, the most likely indicator of melamine poisoning, said Bernadette Burden, a C.D.C. spokeswoman.

How the chemical — found in wheat gluten imported from China — could have poisoned cats and dogs remains something of a mystery, scientists said, because tests done decades ago on rats concluded that it was not very toxic. “Even when rats were loaded up with quite a lot, it caused stones or tumors over time, not acute kidney failure,” said John T. Groves, a Princeton University chemistry professor.

“I’m convinced that melamine can’t do it by itself,” said Richard Goldstein of Cornell University’s veterinary college. Toxicity trials, he said, were done with pure melamine, not manufacturing scrap, which could also contain cyanuric acid or other compounds.

Also, said Jack D. Henion, a retired professor of toxicology and veterinary medicine at Cornell, cats — the first animals whose deaths from pet food were noticed — are unusually sensitive to kidney and urinary problems.

Melamine is a simple organic molecule that can be extracted from oil or coal, and is used as a resin in making plastics. It is found in plates, countertops, cabinet facings, even children’s erasers.

It contains large amounts of nitrogen. For that reason, it has apparently been used as a fertilizer, though it would presumably be harmless because plants would take up only the free nitrogen.

But in China, ground-up scraps from plastic-making have been added to grain and byproducts, like the sticky glutens used to thicken soups or help bread rise. It is presumably done to fool buyers into thinking they are getting a more nutritious product, because crude tests for protein in grain do not test for protein molecules, but for nitrogen, which proteins also have in abundance.

“It’s blatant fraud in my view,” Dr. Henion said. “There’s no other good reason we know of to put melamine in.”

Some of those grain products have been sold to the United States for pet food. Last week, the drug agency said it would start requiring importers to provide proof of the safety of many more ingredients, including glutens and proteins of wheat, rice, corn, soy and mung bean.

Officials from the drug agency and the Agriculture Department estimated yesterday that up to three million young broiler chickens on farms in Indiana ate feed contaminated with pet food containing melamine or related compounds in February.

Most were slaughtered, sold and, presumably, eaten, said Kenneth Peterson, assistant administrator for field operations with the Food Safety and Inspection Service.

Even in those chickens, or in hogs on other farms, independent toxicologists and drug agency and agriculture department officials said, it was highly unlikely that the melamine could concentrate in amounts that could be harmful to humans.

It would be mixed with other feeds, and much of it would be metabolized by animals and excreted. And it is not stored in fat as, for example, pesticides are. Also, most humans are not strictly carnivores, as cats are, so they would get lower doses.

“The dilution levels are enormous,” David Acheson, the drug agency’s assistant commissioner for food protection said yesterday. “When you multiply it all together, we think the likelihood of human illness is very low.”

Two drug agency employees arrived this week in China, an agency official said yesterday.

But the May Day holiday, which lasts all week there, could slow progress, though the Chinese government had agreed to make an official available, said Walter M. Batts, the drug agency’s deputy director for international relations.

David Barboza contributed reporting from Zhangqiu, China, and Alexei Barrionuevo from Chicago.

A version of this article appears in print on , on Page A16 of the New York edition with the headline: Pet Food Chemical Unlikely to Pose Threat to Humans, Experts Say, as U.S. Continues Inquiry. Order Reprints|Today's Paper|Subscribe